A matched filter is a filter for judging the identification of two signals. In spread spectrum communications, a unique spreading code sequence is allocated for each user. Each user finds a signal transmitted for the user by a matched filter to which one spreading code sequence is applied. The matched filter outputs a correlation peak when the signal is the user's for aquisition and holding.
Assuming a spreading code to be PN(i), a tip time to be Tc, a spreading ratio to be M, a time to be (t), an input signal at a time t to be S(t) and a correlational output signal at a time t to be R(t), the following formula (1) can be obtained. ##EQU1## Here, PN(i) is a 1 bit data sequence.
As double or higher order sampling is necessary for the acquisition, the calculation of the formula (1) is performed in a plurality of systems at the same time using a plurality of matched filters and the calculation results are added. In order to realize such a matched filter, a digital circuit or a SAW (Surface Acoustic Wave) device was conventionally used. However, using a digital circuit, the electric power consumption was necessarily large because the size of a circuit was large, therefore, it was not appropriate for mobile communications. Using a SAW element, it was not easy to realize a whole circuit by a single element and the S/N ratio was low.
Since the spreading code is a 1 bit string, the inventors have proposed a matched filter in the Japanese patent application No. 7-212438, which is high-speed and can be implemented as a small size, low power LSI. The sequence of processing is as follows: i) sampling and holding input signals as analog signals of a time series, ii) branching them out into the signal groups "1" and "-1" by a multiplexer, and iii) adding the signals in each group in parallel by a capacitive coupling. However such a matched filter circuit still has large circuit size when a long P/N code was processed making it difficult to introduce into mobile communications.